The invention relates to a method for the electroslag refining of metals in which at least 50% of the metal is in the form of at least one current-carrying consumable electrode, especially one having alloy components with an affinity for oxygen, which is melted through a molten pool of slag to form an ingot.
In the electroslag refining process, the metal starting material is remelted through a liquid or molten slag layer to form an ingot or block on whose top surface a liquid zone or molten pool is maintained. The ingot can be held fixed (in an upright mold) or it can be drawn continuously downward (through a strand casting mold). The starting material can be added both in the form of a consumable electrode and in the form of lumps or particles. The melting and process heat is produced by the electrical resistance of the molten slag, the power being fed either through the consumable electrode or (in the case of particulate starting material) through a special permanent electrode. As a rule the ingot and/or the mold is the electrical counter-pole. It is known to perform the electroslag refining process by means of either direct current or alternating current.
DE-OS No. 14 83 646 discloses the performance of electroslag refining under subatmospheric pressure, i.e., under a pressure of less than 1 bar. In this case permanent electrodes are always provided for the power input.
For the production of castings to satisfy exacting requirements, especially from superalloys for rotors in aircraft engines, the customers require that the ingots be made by the known vacuum refining process (VAR), since refining under a vacuum results in relatively pure ingots having a very low gas content. Despite the fact that in the VAR process the ingots are normally free of macrosegregations due to controlled solidification, a number of typical segregation phenomena occur in the ingots, such as freckling, rings and white spots. While segregation phenomena such as the freckling and rings can be more or less overcome by careful adjustment of the melting parameters, the formation of white spots can occur no matter what the melting conditions are. Recently performed studies have shown that the formation of white spots is not the result of irregular solidification at the solidification front. It can be assumed that the components of the white spots are the following:
skeletons of dendrites which drop from the molten consumable electrode during the melting, PA1 particles which drop down from the so-called "crown" at the upper edge of the ingot (the crown is a thin, sharp edge above the lake due to condensation or solidification of vapors and splashes, PA1 particles that come loose from the solidifying edge of the molten pool. PA1 (a) the refining process is performed under less than atmospheric pressure, PA1 (b) an at least 80 weight-percent oxidic slag is used, composed of oxides whose boiling points are higher than 2000.degree. C., and PA1 (c) the slag is heated by alternating current.
Another source of the white spots can, according to the inventor's own experience, consist of particles which can originate from the cast electrode when the latter consists of a superalloy, which very often splits along the radial crystals. It is therefore very difficult, if not quite impossible, to prevent these flaws in a VAR ingot.
In the electroslag refining process described in the beginning, the refining is performed under a superheated molten slag bath whose temperature is usually more than 300.degree. C. above the liquidus temperature of the superalloy. The dendrite skeleton or the particles that break out of the electrode necessarily drop through the superheated slag and consequently have enough time to melt before they reach the pool. Also, in the electroslag process no crown forms at the top edge of the ingot. Consequently, the electroslag refining process does not lead to the formation of white spots.
Although the ingots produced by the electroslag process are at least as good as those obtained by the VAR process, the purchasers of superalloys regularly call for the use of the VAR process for the production of rotors for aircraft engines. The reason for this is to be seen in the fact that, in the conventional electroslag processes not only does no degassing of the material occur, but in certain cases an additional absorption of gas is to be feared, and hydrogen and nitrogen are the most dangerous gases.
Another very important danger consists in the formation of oxides and oxidic inclusions by the oxidation of the metal, especially of the alloy components which have an affinity for the oxygen in the ambient air. These alloy components with an affinity for oxygen are the elements aluminum, boron, titanium, zirconium and others. The oxidation of such alloy components then results in a defect.
The invention is therefore addressed to the problem of devising a process of the kind described in the beginning, in which oxidation is prevented, degassing takes place, and neither freckles nor ring patterns nor white spots occur. It is important to note that all aspects of the problem in question are solved simultaneously.